The Focus: Thyroid eye disease (TED) is a disfiguring and sight-threatening autoimmune disease that involves inflammation and remodeling of the orbit. TED can present itself with mostly orbital fat (Type 1), orbital connective tissue (Type 2) or a combination of both. Here, we focus on how TED manifests itself as Type 1 disease. The Premise: We know that activated orbital fibroblasts produce high levels of inflammatory cytokines and form either connective tissue myofibroblasts or lipid rich inflammatory adipocytes. Our earlier work showed - that only human orbital fibroblasts that do not express a cell surface protein called Thy1 (Thy1 ) can differentiate into adipocytes when stimulated with an adipogenic medium. And, only those that express Thy1 (Thy1+) differentiate into myofibroblasts when provoked by pro-scarring agents like TGF?. Herein, we show that Thy1 presence has a direct role in diminishing orbital fibroblast fat accumulation. For example, deliberate - expression of Thy1 in Thy1 fibroblasts blocks their differentiation to adipocytes. Since adipogenesis requires the activation of the transcription factor PPAR?? we propose that Thy1 impairs PPAR? function. Adipogenesis can also manifest after thyroid stimulating hormone receptor (TSHR) signaling and our studies demonstrate that Thy1 and TSHR are inversely related. Goals: (a) test specific hypotheses about mechanistic and causal roles of Thy1 in TED, (b) determine the molecular mechanisms by which Thy1 expression dampens adipogenesis and inflammatory cytokine production. These mechanisms include [1] regulation of TSHR, [2] PPAR? activity and [3] modulation of post-transcriptional gene expression through key microRNAs. Organizing Hypotheses: Thy1 is more than a cell-surface marker in TED, as it is also a functional mediator of fibroblast fate in the disease. Thy1+ orbital fibroblasts have decreased PPAR? activity, decreased TSHR, and decreased miR-130a expression, which inhibit the fundamental processes in TED of orbital fibroblast adipogenesis and proinflammatory cytokine production. The proposed experiments will test corollaries of this hypothesis using primary human orbital fibroblasts and tissues; because there is no animal model that provides a consistent phenotype of fat accumulation in the orbit. Specific Aim 1: Test the hypothesis that orbital fibroblast Thy1 has anti-adipogenic and anti-inflammatory mechanisms of action by inhibiting PPAR? activity and regulating post-transcriptional gene expression. Specific Aim 2: Test the hypothesis that the mechanism by which Thy1 has its anti-adipogenic and anti-inflammatory effects is through the regulation of TSHR. Impact on the field: Discovering that Thy1 has a major biological function influencing eye disease is a major advance opening new areas for research and therapeutic intervention into the aberrant fat deposition and inflammation associated with TED. The inverse relationship that will be investigated between Thy1 and TSHR is the first demonstration that Thy1 can control TSHR. The results will lead to further research on how targeting of Thy1 and/or its anti-adipogenic mechanism of action could be used in more effective therapeutics.